Solid state imaging devices, e.g., CCD, CMOS, and others, include a lens or series of lenses to direct incoming light onto a focal plane array of pixels. Each one of the pixels includes a photosensor, for example, a photogate, photoconductor, or photodiode, overlying a substrate for accumulating photo-generated charge in an underlying portion of the substrate. The charge generated by the pixels in the pixel array is then read out and processed to form an image.
Lenses used to direct light onto a pixel array may cause an optical aberration known as field curvature. Field curvature is a simple lens aberration where the sharpest focus of the lens is on a curved surface, also known as a Petzval surface. Field curvature can be understood as a defocusing with field angle, (i.e., bundles of light having different chief ray angles will be focused at different distances from the lens). Therefore, objects in the center and edges of an image are never perfectly in focus simultaneously.
FIGS. 1A, 1B, and 1C are diagrams of a simple lens 100 focusing a bundle of light rays 140A, 140B, 140C (collectively referred to as “light rays 140”) onto a pixel array 130. Each of the bundles of light rays 140A, 140B, 140C, has a different chief ray angle CRA and are each optimally focused at a curved focal surface 120. However, since the pixel array 130 is a flat, (i.e., plano), surface, light rays 140B, 140C having larger chief ray angles CRA striking the edge of the pixel array 130 are more unfocused and will appear more blurry in a produced image. The captured image appears sharp and crisp in the center of the image, but is blurry at the edges.
What is needed is a system and method by which light rays are better focused onto a planar pixel array of an imaging device.